May have already written about this in one of my pi 101 posts. But here’s a simple command to find your pi if you accidentally boot it with no screen and you want to ssh in.
First make sure you have this: http://nmap.org/download.html
Then use this:
sudo nmap -sP 10.3.1.1-255 | grep raspberry
That should turn up the network IP address your pi on your subnet (NOTE: you may have to use 192.168.1.1-255) if you are working from a normal network.